World's smallest working fuel cell

January 26, 2009 People are becoming more and more reliant on the portable gadgets they carry every day and at no time is that reliance more pronounced as when those gadgets’ batteries run out of juice. While we’ve seen some fuel cell technologies appear in recent years that offer the future prospect of powering portable devices, none are quite as small as this new working fuel cell created by US chemical engineers and featured recently in New Scientist that measures just 3 millimeters across.

Although fuel cells are able to store more energy than batteries in the same space, batteries continue to reign supreme as they are much easier to produce at the small scale than the pumps and control electronics required in a fuel cell. The pumps in such devices can also use more energy than they generate making them impractical. Now a team at the University of Illinois at Urbana-Champaign has overcome this problem by designing for a tiny fuel cell that generates power without consuming it.

The fuel cell works by allowing water molecules from the water reservoir to reach an adjacent chamber containing metal hydride as vapor through tiny holes in a thin membrane. Once there, the vapor reacts with the metal hydride to form hydrogen, which fills the chamber, pushing the membrane upwards and blocking the flow of water. The resultant hydrogen reacts with an assembly of electrodes beneath the metal hydride chamber to create a flow of electricity as it is gradually depleted.

Since the device is so small it can rely on surface tension, not gravity, to control the flow of water through the system, meaning that the cell will operate even if moved and rotated. Also, since the new device carries its fuel internally, when measured as a volume, the power density of the new fuel cell is a relatively high 100 watts per liter. The first designs generated 0.7 volts and a current of 0.1 milliamps for 30 hours before the fuel ran out, but the team says the latest designs give currents of around 1 milliamp at a similar voltage. While that's not yet enough to drive mobile phones or MP3 players, which use batteries typically rated at a few volts, the prospect of such fuel cells being used to power greener gadgets in the future is definitely promising.